My husband and I are going on a trip to Hawaii and I have just enough miles for us to go 1 way economy class and 1 way first class. I was thinking about 1st class on the way home because we'll be so excited on the way there that the upgrade wouldn't be as big of a deal.

Is it worth it to spend another 40K miles between the two of us to upgrade on United Airlines KOA-SFO? I was trying to figure it out on the UA website what kind of benefits we'd get in first class, but I'm not sure if it would be domestic even though it's a long flight about 5 hours. We'd either be flying on a 757 at 9pm (red eye) or 767 at noon.

Thanks for any advice you can give!